This invention relates to an improved process for preparing oxidation catalysts, and its use in a process for the preparation of carboxylic acid anhydrides from hydrocarbons. More particularly, it relates to a novel and simpler method for the production of vanadium-phosphorus-oxygen catalyst composites by a heterogeneous solution reducing method providing increased yields. Still more particularly, it relates to the production of maleic anhydride from n-butane, or n-butene, in a vapor phase process employing catalyst prepared by the process of the present invention.
Methods for the preparation of catalyst compositions of vanadium, phosphorus, and oxygen, and the use of these compositions as catalysts in hydrocarbon oxidations are known in the art.
Such preparative methods can be generally categorized as being aqueous-based or organic-based and employ either homogeneous solutions and/or heterogeneous mixtures (e.g., suspensions) of at least one of the components (e.g., a vanadium containing compound) whlch eventually forms the catalyst composition.
The particular method of preparation selected will depend on the various combination of properties sought to be imparted to the catalyst and the commercial attractiveness of the process. Particularly significant properties sought to be influenced by the catalyst preparative methods of the prior art include the vanadium valence, the P:V atomic ratio, the crystal phases of the catalyst, and the surface area of the catalyst.
While at least one patent seeks to impart a vanadium valence of less than +3.9, namely, U.S. Pat. No. 4,178,298, a majority of patents seek to obtain a vanadium valence between +4 and +5.
One preferred way of achieving this is to begin with vanadium in the +5 valence state and reduce the valency to less than +5, or alternatively to start with a vanadium compound having a valency of less than +5. A wide variety of reducing agents can be employed for the former reducing method approach. Representative of such reducing agents include acids such as hydrochloric, hydroiodic, hydrobromic, acetic, oxalic, malic, citric, formic and mixtures thereof such as a mixture of hydrochloric and oxalic may be used. Sulphur dioxide may be used. Less desirably, sulfuric and hydrofluoric acids may be employed. Other reducing agents which may be employed are organic aldehydes such as formaldehyde and acetaldehyde; alcohols such as pentaerythritol, diacetone alcohol and diethanol amine. Additional reducing agents include hydroxyl amines, hydrazine, nitric acid, and the like.
Reducing methods also can be classified according to whether the vanadium compound is dissolved, e.g., solution reducing methods, or not, e.g., heterogeneous reducing methods.
In accordance with solution reducing methods, a vanadium compound having a valence of +5 such as V.sub.2 O.sub.5 is dissolved in a solution containing the reducing agent. Because many strong acid reducing agents, such as HCl, also function to dissolve the vanadium compound, and, therefore, act as a solvent, the solvent and reducing agent can be the same (see for example Kerr, U.S. Pat. No. 3,288,721). Thus, a strong acid reducing agent (e.g., HCl) can be employed in an aqueous or non-aqueous (e.g., organic) medium to achieve dissolution and reduction therein. The phosphorus compound can be added to the vanadium compound for reaction therewith before or after vanadium reduction takes place to form the V--P--O catalyst precursor.
After the aforedescribed reducing methods are employed and the V--P--O catalyst precursor is formed, it is conventional to subject the resulting precursor to some type of activation procedure. The particular set of activation conditions which are employed depends on the initial treatment procedures employed in the preparation of the precursor to be activated.
Thus, it is conventional to activate vanadium-phosphorus-oxygen containing compositions prepared by the aqueous-based solution reduction method by contacting the same with a reducing atmosphere such as CO, H.sub.2, H.sub.2 S and in the essential absence of added gaseous oxygen at temperatures of about 300.degree. to 600.degree. C. (see for example U.S. Pat. Nos. 4,062,802 and 4,122,096). Other activation methods applied to compositions prepared by aqueous-based solution reduction methods include: calcining in an inert atmosphere such as CO.sub.2, N.sub.2, a noble gas, or butane free oxygen (U.S. Pat. Nos. 3,907,707, and/or 4,178,298, and/or 4,111,963); calcining in oxygen (e.g., air) and then an inert atmosphere, e.g., N.sub.2 or noble gas, (U.S. Pat. No. 3,977,998); calcining in air or an oxidizing gas alone (U.S. Pat. Nos. 3,907,707 and 4,111,963 where P:V atomic ratio is greater than 1:1; 3,915,892; and 4,179,404); and heating in a gaseous mixture containing air and a reducing component, e.g., butane (U.S. Pat. No. 3,915,892).
Activation methods for vanadium-phosphorus-oxygen containing compositions prepared by organic-based solution reduction methods can be conducted by heating in air alone and then a gaseous mixture of air and butane. An illustration of this type of activation procedure is found in U.S. Pat. Nos. 3,864,280 and 4,017,521. Activation in air alone is disclosed in U.S. Pat. No. 4,179,404. U.S. Pat. No. 4,043,943 discloses an organic-based solution reduction method wherein a vanadium phosphate compound is precipitated from a liquid organic medium and activated. While it is broadly disclosed (col.10, lines 16 to 23) that the average vanadium valence can be varied somewhat by oxidative or reductive treatment after precipitation as by subjecting the precipitated solid to an oxidizing or reducing atmosphere, these treatments are a less preferred substitution for the use of a suitable oxidizing or reducing reagent to impart a vanadium valence of 3.8 to 4.6, and do not constitute or perform the function of activation. The activation procedure is described at col. 2, lines 59-62, i.e., calcination at a temperature of 100.degree. to 500.degree. C., and at col. 12, lines 29 to 32, i.e., in an atmosphere of air alone and then a mixture of air and butane.
Thus, none of the prior art appears to be directed to preparing a V--P--O containing composition by an organic-based solution reduction method followed by activation in an atmosphere which excludes air alone at any time during the activation procedure.
Similar observations can be made with respect to conventional organic-based heterogeneous reduction methods. Organic-based heterogeneous reduction methods can be classified into those which reduce the vanadium compound in an organic slurry prior to or after contact with the phosphorus compound. For example, U.S. Pat. Nos. 4,132,670 and 4,187,235, which both contain essentially the same disclosure, are directed to an organic-based heterogeneous suspension type reduction method wherein V.sub.2 O.sub.5 is first reduced with a suitable liquid organic medium, e.g., isobutanol, to impart a vanadium valence of between 4.0 and 4.6, and subsequently contacting the reduced vanadium compound with, for example, orthophosphoric acid to form a heterogeneous slurried reaction mixture of a suspended vanadium (IV) phosphate composition. This composition is recovered and calcined, i.e., activated, at about 380.degree. C. by contact with a stream of air alone, and then a gaseous mixture of air and butane. The performance of the catalyst is disclosed as being severely dependent on this activation procedure (col. 7, lines 55-61 of U.S. Pat. No. 4,132,670). The present invention is directed to an improvement in this process.
European Patent Application Publication No. 0039 537, published Nov. 11, 1981 and based on U.S. Pat. application Ser. No. 146,971 filed May 5, 1980, discloses an organic heterogeneous slurry reducing method wherein a pentavalent vanadium compound and phosphorus compound are admixed prior to reducing the vanadium valence with, for example, isobutanol. The resulting catalyst precursor is calcined in air or an oxygen containing gas at a temperature of 250.degree. to 600.degree. C. Not only does this application fail to disclose the advantage of excluding air alone as an activation or calcination atmosphere, but as described in Comparative Example 2, the catalyst of this application performs better when activated in air alone relative to the exclusion of air alone in favor of an air and butane mixture.
U.S. Pat. No. 4,317,778 discloses a variety of V--P--O catalyst preparative methods, including aqueous and organic, solution and heterogeneous, reduction methods, all of which require the use of specific ratios of orthophosphoric and pyrophosphoric acids as the source of the phosphorus compound to minimize the solubility of the catalyst in water and thereby to enable the catalyst to be spray dried. The catalyst precursor is calcined in air or an oxygen containing gas at temperatures of 250.degree. to 600.degree. C. The catalyst precursor may also be calcined "either in the presence of hydrocarbon, in an inert gas, or both," (col.6, lines 50 et seq.). This patent, however, does not disclose the particular combination of catalyst precursor preparative method and activation procedure of the present invention.
U.S. Pat. No. 3,985,775 discloses organic and aqueous solution reduction methods using HCl, as well as an organic heterogeneous method wherein vanadium is reduced and reacted with a phosphorus containing compound while the vanadium compound is suspended in an organic solvent (e.g., THF, example 14) to form a dihydrate precursor. However, it is suggested therein that even when employing an organic-based method, as much as 20 to 40% by weight of the liquid medium can be water (col. 6, lines 21-23). Furthermore, while several general classes of liquid organic media are disclosed, i.e., alcohols, ethers, and carboxylic acids, no preference for alcohols is expressed, nor is there any exemplification of the use of any alcohol in any organic based procedure. Numerous, i.e., eight, different complicated activation, i.e., pretreatment, procedures are also disclosed, only one of which avoids contact of the dihydrate with air alone, namely, pretreatment method H. However, pretreatment method H is only applied to an aqueous-based solution reduced dihydrate composition. The pretreatment procedure applied by exemplification to an organic-based heterogeneously reduced dihydrate, employs contact with air alone and then air and butane (example 14). Moreover, the results obtained using pretreatment H are inferior to a majority of the other pretreatment methods, and these results are reported after 530 hours run time, more than twice the run time of any other catalyst prepared by other pretreatment methods. Thus, this reference provides no suggestion of selecting the operative preparative variables to include an alcohol as the liquid medium, a heterogeneous treatment of vanadium to reduce the vanadium in the absence of a phosphorus compound, reaction of the reduced vanadium compound with a phosphorus compound in a heterogeneous reaction mixture, and activation of the resulting product in a hydrocarbon-air atmosphere.
U.S. Pat. No. 3,975,300 is directed to a one-step method for preparing vanadium-phosphorus composites wherein a paste comprising an organic reducing agent, e.g., ethylene glycol, phosphoric acid, and a vanadium compound is formed and then evaporated to dryness. The dried composition is then optionally conditioned in a hydrocarbon-air mixture at about 450.degree. C. This process differs from the process of U.S. Pat. No. 4,132,670 in that reduction of vanadium takes place in the presence of phosphoric acid and a slurry or suspension of the components of the paste is never prepared.